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http://en.wikipedia.org/wiki/PH
pH
 From Wikipedia, the free encyclopedia
"PH" redirects here. For other uses, see PH (disambiguation).

In chemistry, pH is a measure of the acidity or basicity of an aqueous
solution.[1] Pure water is said to be neutral, with a pH close to 7.0 at
25 °C (77 °F). Solutions with a pH less than 7 are said to be acidic and
solutions with a pH greater than 7 are basic or alkaline. pH
measurements are important in medicine, biology, chemistry, agriculture,
forestry, food science, environmental science, oceanography, civil
engineering and many other applications.

In a solution pH approximates but is not equal to p[H], the negative
logarithm (base 10) of the molar concentration of dissolved hydronium
ions (H3O+); a low pH indicates a high concentration of hydronium ions,
while a high pH indicates a low concentration. This negative of the
logarithm matches the number of places behind the decimal point, so, for
example, 0.1 molar hydrochloric acid should be near pH 1 and 0.0001
molar HCl should be near pH 4 (the base 10 logarithms of 0.1 and 0.0001
being −1, and −4, respectively). Pure (de-ionized) water is neutral, and
can be considered either a very weak acid or a very weak base (center of
the 0 to 14 pH scale), giving it a pH of 7 (at 25 °C (77 °F)), or
0.0000001 M H+.[2] For an aqueous solution to have a higher pH, a base
must be dissolved in it, which binds away many of these rare hydrogen
ions. Hydrogen ions in water can be written simply as H+ or as hydronium
(H3O+) or higher species (e.g., H9O4+) to account for solvation, but all
describe the same entity. Most of the Earth's freshwater bodies surface
are slightly acidic due to the abundance and absorption of carbon
dioxide;[3] in fact, for millennia in the past, most fresh water bodies
have long existed at a slightly acidic pH level.

However, pH is not precisely p[H], but takes into account an activity
factor. This represents the tendency of hydrogen ions to interact with
other components of the solution, which affects among other things the
electrical potential read using a pH meter. As a result, pH can be
affected by the ionic strength of a solution—for example, the pH of a
0.05 M potassium hydrogen phthalate solution can vary by as much as 0.5
pH units as a function of added potassium chloride, even though the
added salt is neither acidic nor basic.[4]

Hydrogen ion activity coefficients cannot be measured directly by any
thermodynamically sound method, so they are based on theoretical
calculations. Therefore, the pH scale is defined in practice as
traceable to a set of standard solutions whose pH is established by
international agreement.[5] Primary pH standard values are determined by
the Harned cell, a hydrogen gas electrode, using the Bates–Guggenheim
Convention.

pH in its usual meaning is a measure of acidity of (dilute) aqueous
solutions only.[1] Recently the concept of "Unified pH scale"[6] has
been developed on the basis of the absolute chemical potential of the
proton. This concept proposes the "Unified pH" as a measure of acidity
that is applicable to any medium: liquids, gases and even solids.

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http://en.wikipedia.org/wiki/Ion
(see image)
Hydrogen atom (center) contains a single proton and a single electron.
Removal of the electron gives a cation (left), whereas addition of an
electron gives an anion (right). The hydrogen anion, with its loosely
held two-electron cloud, has a larger radius than the neutral atom,
which in turn is much larger than the bare proton of the cation.
Hydrogen forms the only cation that has no electrons, but even cations
that (unlike hydrogen) still retain one or more electrons, are still
smaller than the neutral atoms or molecules from which they are derived.